Combustors with combustion of a fuel, usually coal, in a fluidized bed are advantageously constructed with a so-called open bottom consisting of parallel air distributing tubes for combustion air to a combustion space above the bottom and with one or--in large combustors--several funnel-shaped ash chambers below the air distributing bottom tubes. Residual products, residues from the fuel and consumed bed material absorbent, pass in gaps between the air distributing tubes to the ash chamber or ash chambers. The gaps between the tubes should be of such a size that slag lumps formed during normal operation may pass freely through the gaps so as not to disturb the combustion. It is desirable that the downwardly-directed flow to the ash chamber is evenly distributed over the cross section of the combustor. To obtain a small overall height and a uniform material flow, therefore, large combustors are formed with a plurality of ash chambers with a rectangular cross section nearest the combustor bottom. The ash chambers may have the shape of a pyramid turned upside down, or of a funnel of rectangular cross section which changes into a circular cross section so that its lowermost part has the shape of a cone. A power plant with a combustor of this kind is described in greater detail in European patent application A1 289 974.
Between the ash chambers and the surrounding space in the pressure vessel, the pressure difference may amount to about 1 bar, which means that they are subjected to great forces. Since the temperature is high also in the ash chamber, especially in the uppermost part, the ash chamber walls have been constructed as cooled panel walls which are traversed by cooling water to attain a satisfactory strength. Constructing the ash chambers with water-cooled panel walls in a combustor with a plurality of ash chambers complicates the design and entails high costs of manufacture and erection.